The immediate answer: diff -N, as explained by pootzko. You'll find that a lot of patches out there are created by diff -urN.

What can make your life better: start using a version control tool. If you don't know any, start with one of the three main distributed revision control systems, Bazaar, Git or Mercurial. Check in the clean version, work, check in your work as many times as you like, and ask your version control system for a diff between the clean version and your work.